As well known, cathode-ray tubes are widely used in various types of television equipment such as television receivers and television display units, and the deflection yoke is employed in each cathode-ray tube to deflect at least one electron beam.
The deflection yoke has a pair of horizontal deflection coils and a pair of vertical deflection coils. The horizontal deflection coils generate a horizontal deflection magnetic field to deflect at least one electron beam in the horizontal direction when a horizontal deflection current of approximately 15.75 kHz to 120 kHz is supplied. The vertical deflection coils generate a vertical deflection magnetic field to deflect at least one electron beam in the vertical direction when a vertical deflection current of 60Hz or 50Hz is supplied.
These deflection magnetic fields are distributed as a leaking magnetic field both inside and outside the deflection yoke. Of these magnetic fields, the leaking magnetic field generated inside the deflection yoke contributes to deflection of the electron beam and the attention has been paid to improvement of this internal magnetic field. On the other hand, the leaking magnetic field radiated outside the deflection yoke, that is, the externally leaking magnetic field, does not greatly affect the characteristics of the deflection yoke and the function of the deflection yoke is based on the utilization of the leaking magnetic field. Accordingly, up to now, measures to reduce the externally leaking magnetic field have rarely been taken.
Lately, various types of personal computers and electronically controlled office machines have been widely used at various offices and job shops where they are used in many cases in the vicinity of television equipment. Therefore, externally leaking magnetic fields of the deflection yoke, which had not been a problem, have a high frequency magnetic field generated from the horizontal deflection coils. The magnetic field has been considered to affect other electronic equipment as electromagnetic interference which causes these electronic equipment to malfunction.
For this reason, in some deflection yokes, a magnetism shielding cylinder which surrounds the deflection yoke is provided to reduce such electromagnetic interference.
However, provision of the magnetism shielding cylinder around the deflection yoke is disadvantageous in that a larger space will be required to result in a large size design of the housing of the television equipment. Also, the magnetism shielding cylinder forms the magnetic path for the externally leaking magnetic field to adversely affect landing and convergence of electron beams.